1. Field of the Invention
This invention relates to a variable magnification image formation apparatus which uses a zoom lens assembly to form an image of an original on a photosensitive member.
2. Description of the Prior Art
A variable magnification image formation apparatus using a lens of variable focal length, namely, a so-called zoom lens assembly, has various advantages over a variable magnification image formation apparatus using a so-called fixed focus lens assembly. For example, in apparatus using a fixed focus lens assembly for changing magnification, it is necessary to move not only the lens but also a mirror, since it is necessary to vary the ratio of the lengths of the optical path before and behind the lens as well as the length of the entire optical path between the original and the surface of the photosensitive member in the apparatus using a zoom lens assembly, for changing magnification, it is not necessary to vary the length of the entire optical path so that it is not necessary to move the lens. Therefore, in apparatus using a zoom lens assembly, the construction of the magnification changing mechanism is relatively simple and the apparatus is compact. Also, the number of movable elements can be reduced and this leads to the possibility of forming images of good quality.
In the above-described image formation apparatus using a zoom lens assembly, during magnification change, the zoom lens assembly must be moved while, at the same time, the focal length of the zoom lens must be varied. Apparatus in which the focal length of the zoom lens is varied are disclosed in U.S. Pat. Nos. 3,967,896; 4,046,467; and 3,765,760 and Japanese Laid-open Patent Application No. 133756/1981. These literatures only disclose apparatus in which one lens barrel portion is moved back and forth relative to the other lens barrel portion by a cam of a lever provided adjacent to the lens barrel, and do not suggest a focal length changing device of a zoom lens assembly of the type in which the focal length is changed by a member rotated about the optical axis of the lens, such as a so-called zoom ring.